1. Field of the Invention
The present invention relates to an apparatus for and a method of recording a two-dimensional image on a recording sheet with a recording head, and more particularly to an image recording apparatus preferably for use as an external-surface-scanning image recording apparatus for recording an image on a recording sheet such as a printing plate or the like on the outer circumferential surface of a drum which is being rotated, and a method of recording an image with such an image recording apparatus.
2. Description of the Related Art
Heretofore, it is known that a cylindrical drum is rotated at a constant speed and a recording sheet such as a photosensitive sheet mounted on the outer circumferential surface of the rotating drum is scanned in a main scanning direction with a light beam that is emitted from a recording head and also in an auxiliary scanning direction by moving the recording head along the axis of the drum for thereby recording a two-dimensional image on the surface of the photosensitive sheet (see, for example, Japanese laid-open patent publication Nos. 5-207250, 9-149211, and 10-16290).
FIG. 21 of the accompanying drawings shows a general processing sequence of such an image recording apparatus.
In step S1 shown in FIG. 21, a photosensitive sheet is supplied and loaded onto the drum, and mounted on the outer circumferential surface of the drum.
In step S2, the photosensitive sheet mounted on the drum is exposed to light. The exposed photosensitive sheet is then unloaded and discharged from the drum in step S3, and supplied to a next process.
FIG. 22 of the accompanying drawings shows details of the exposure process in step S2 shown in FIG. 21.
When the exposure process is started, a drum motor for rotating the drum is energized in step S2A. Then, it is determined in step S2B whether the rotational speed of the drum has reached a constant rotational speed and has been stabilized at the constant rotational speed or not.
If the rotational speed of the drum has been stabilized, then a PLL (Phase-Locked Loop) circuit is energized with pulses from an encoder mounted on the shaft of the drum in step S2C. In step S2D, it is determined whether the phase-locked loop is stabilized or not based on an output signal from the PLL circuit.
If the phase-locked loop is stabilized, then the recording head is moved in the auxiliary scanning direction in step S2E. In step S2F, image data read in synchronism with a recording clock generated from the output signal from the PLL circuit is supplied to the recording head, which emits a light beam modulated in intensity by the image data. The emitted light beam is applied to scan the photosensitive sheet in the main scanning direction, thus exposing the photosensitive sheet to the light beam.
If the end of the exposure of the photosensitive sheet for one image is confirmed in step S2G, the exposure of the photosensitive sheet to the light beam is stopped in step S2H, the movement of the optical system in the auxiliary scanning direction is stopped in step S2I, followed by returning the recording head to its home position in the auxiliary scanning direction in step S2J, and the drum motor is de-energized in step S2K. In this manner, the exposure process in step S2 is put to an end.
As can be understood from the operation of the conventional image recording apparatus in steps S2B through S2G, the exposure of the photosensitive sheet to the light beam to record image information on the photosensitive sheet (writing or drawing process) is started after the drum reaches a constant rotational speed and its rotation is stabilized, i.e., after the drum enters a constant-speed rotating state.
In order to increase the productivity in the exposure recording process time (cycle time) for the photosensitive sheet in recent years, it has been proposed to increase the rotational speed (constant rotational speed) of the drum from the conventional speed ranging from 200 to 300 rpm to a speed of 1000 rpm or higher.
If the rotational speed of the drum in the exposure recording process is to be increased, then the startup time required for the drum to reach the high rotational speed and the shutdown time required for the drum to fall from the high rotational speed to a stop are increased. Though the overall operating time of the image recording apparatus from the start of drum rotation to the stop of drum rotation is shortened, the effect that the startup time and the shutdown time have on the productivity is non-negligibly large.
As the drum is rotated at a higher rotational speed, the drum produces larger centrifugal forces. Therefore, the wall thickness of the drum needs to be increased in order to avoid a distortion of images due to physical irregularities such as eccentricity of the drum. If the drum has an increased wall thickness, however, the drum becomes heavier and suffers a greater inertial moment. Consequently, the drum motor needs to produce a larger power output so as to make the startup time and the shutdown time equivalent to those in the conventional recording apparatus.
However, the drum motor with the higher power output is problematic in that its electric power consumption, cost, volume, and weight are increased.
It is therefore an object of the present invention to provide an apparatus for and a method of recording an image while minimizing a loss of a recording time due to a startup time and a shutdown time for the rotation of a drum and increasing the productivity (cycle time) of a recording process for recording an image on a recording sheet at a low cost.
Another object of the present invention is to provide an apparatus for and a method of recording an image while increasing the productivity of a recording process even if a motor with the same power output as is conventional is used.
According to the present invention, a recording head records an image on a recording sheet held on an outer circumferential surface of a drum in at least one of an accelerating period in which a rotational drive source accelerates the drum from a stop to a constant rotational speed and a decelerating period in which the rotational drive source decelerates the drum from the constant rotational speed to a stop, and in a constant-rotational-speed period in which the rotational drive source rotates the drum at the constant rotational speed. Therefore, even though the accelerating period and the decelerating period are time-consuming, since the image is recorded in these periods, the productivity of image recording is increased. The recording sheet may comprise a printing plate (direct plate) or a sheet-like photosensitive material such as a PS plate or the like.
The apparatus further comprises recording position correcting means for correcting the position in which the recording head records the image on the recording sheet depending on the peripheral speed of the drum in at least one of the accelerating period and the decelerating period, and recording output correcting means for correcting a recording output from the recording head depending on the peripheral speed of the drum. With this arrangement, the linearity which is an element indicative of the accuracy of the image is prevented from being degraded in the accelerating period and/or the decelerating period.
The recording position correcting means may be arranged to change the speed of the recording head moved by the auxiliary scanning direction moving means.
The recording position correcting means may be arranged to change the frequency of a main scanning write clock signal which is generated depending on the rotational speed of the drum.
The recording position correcting means may be arranged to change the frequency of the main scanning write clock signal by incorporating a variation corresponding to a change in the rotational speed of the rotational drive source into a PLL circuit which detects the rotational speed of the drum and generates a main scanning write clock signal.
The recording position correcting means may be arranged to have a delay circuit connected to an output terminal of a PLL circuit for delaying a predetermined delay depending on the rotational speed of the rotational drive source in the accelerating period or the decelerating period for generating a delayed main scanning write clock signal.
The recording position correcting means may be arranged to start correcting the position in which the recording head records the image on the recording sheet when the rotational speed of the drum becomes equal to or higher than a predetermined speed in the accelerating period.
The recording position correcting means may be connected to a recorded-amount-of-image detecting means for detecting an amount of image to be recorded on the recording sheet, and may be arranged to start correcting the position in which the recording head records the image on the recording sheet in the decelerating period when the recorded-amount-of-image detecting means detects that a remaining amount of image to be recorded reaches a predetermined amount.
According to the present invention, there is also provided a method of recording an image with an image recording apparatus for recording a two-dimensional image on a recording sheet held on an outer circumferential surface of a drum rotated by a rotational drive source by scanning the recording sheet with a recording head in a main scanning direction perpendicular to an axis of the drum and moving the recording head with an auxiliary scanning direction moving means in an auxiliary scanning direction extending along the axis of the drum which is substantially perpendicular to the main scanning direction, the method comprising the step of recording an image on the recording sheet held on the outer circumferential surface of the drum with the recording head in at least one of an accelerating period in which the rotational drive source accelerates the drum from a stop to a constant rotational speed and a decelerating period in which the rotational drive source decelerates the drum from the constant rotational speed to a stop.
With the above method, since an image is recorded in the accelerating period and the decelerating period as well as in the constant-rotational-speed period, the productivity of recording the image on the recording sheet is increased.
The apparatus for and the method of recording an image according to the present invention are preferably applicable to a light beam image recording apparatus for recording an image with a light beam outputted from the exposure head to the recording sheet.
The apparatus for and the method of recording an image according to the present invention are also preferably applicable to an external-surface-scanning ink-jet image recording apparatus for recording an image by applying an ink from an ink jet recording head to the recording sheet.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.